To maximize neutron economy, spacer grids for nuclear fuel assemblies are preferably made from zirconium alloy. Zirconium alloy, however, tends to grow during normal operation of the fuel assembly due to the effects of irradiation and hydrogen absorption. The direction and amount of growth depends on several factors including the cold work, alloy, and texture of the zirconium alloy. A strip of zirconium alloy will tend to grow in the direction of rolling as opposed to the direction of the mill roller axis.
Since grid straps are typically rolled along their longitudinal axis, growth will occur in a “lateral” direction, which is perpendicular to the longitudinal axes of the fuel rods that are supported within the spacer grid cells. This growth contributes to the formation of gaps or empty space between the clad support structures (the springs and dimples) and the cladding. Fuel rod clad creep down, which occurs when the cladding collapses inward on the fuel pellets, and loss of pre-load due to annealing of the zirconium alloy, may also contribute to the formation of gaps. Gapped cells lead to a phenomenon known as grid-to-rod fretting, which occurs when flow around the fuel rods induces vibration, causing the cladding to wear against the clad support structures of the spacer grid. Grid-to-rod fretting may have a detrimental effect on the fuel design's capability to withstand loads, particularly Condition III and IV loads.
One way to mitigate gap formation is to design a spacer grid that includes relatively high growth springs and relatively low growth grid straps. High growth springs will tend to maintain contact with the fuel rod cladding while low growth grid straps will tend to maintain structural stability of the fuel assembly. Several approaches have been developed to achieve such a spacer grid design. In one approach, the spring is stamped from the grid strap material, and greater cold work is imparted to the spring rather than the grid strap. In theory, this approach should result in the spring growing at a faster rate than the grid strap. However, post-irradiation examination (PIE) data has proven otherwise. Another approach to mitigating gap formation involves minimizing the growth of the entire spacer grid (including both the grid straps and the clad support structures) via alloy and strap processing changes. However, this approach is not ideal because it is difficult to achieve the appropriate balance between grid strap and spring growth. A further approach to mitigating grid formation involves a “bi-metallic” grid design in which spring clips made from stainless steel or a nickel-chromium-iron alloy (e.g., Inconel®) are attached to zirconium alloy grid straps. However, this approach is not ideal because steel and nickel-chromium-iron alloys are high parasitic loss materials that tend to absorb neutrons, and it is difficult to ensure the mechanical integrity of the attachment between the two dissimilar metals. Yet another approach to mitigating gap formation involves rolling the grid strap perpendicular to its longitudinal axis (the lateral direction) such that the high growth direction is axially aligned with the longitudinal axes of the fuel rods. The springs are then formed by transverse stamping the grid strap along the direction of growth, parallel to the fuel rods. This approach, however, is not ideal because it is difficult to roll the grid straps in this direction. Also, because the spring growth is proportionally reduced along with that of the base strap, the desired relative growth is not improved over designs stamped in the longitudinal direction.
Thus, there exists a need for a more effective way of mitigating the formation of gaps between fuel rod cladding and clad support structures on a fuel assembly spacer grid.